In recent years, the popularity of portable and cordless electronic devices has rapidly increased. Accordingly, there has also been a great increased in the desire for small and light nonaqueous electrolyte secondary batteries having a high energy density to use as a driving power source of such an electronic device. The development of techniques for nonaqueous electrolyte secondary batteries, not only for electronic devices but also for power storage or electric vehicles with longer durability or the like, has accelerated.
In order to obtain longer durability and other improvements, nonaqueous electrolyte secondary batteries free from internal short-circuits, voltage drop defects, and the like are desirable, with some attention focusing on prevention of the immixing of metallic foreign bodies during the process of manufacturing such a nonaqueous electrolyte secondary battery has been pointed out. The possibility that immixed metallic foreign bodies may melt and be disposed within a battery in such a manner as to penetrate a separator and cause a short-circuit has been noted.
Conventionally, in order to avoid internal short-circuiting, voltage drop defects, and the like in the secondary batteries, it has been assumed that metallic foreign bodies are immixed in nonaqueous electrolyte secondary batteries and pre-shipment preparation processes for foreign body removal and the like have been performed.
Japanese Patent Laid-open Publication No. 2005-158643 (Patent Document 1) discloses, as a method for testing a lithium ion secondary battery, which is a nonaqueous electrolyte secondary battery in which reliable and quick discovery of defective batteries, a method in which a lithium ion battery is held in an environment in which the temperature is 45° C. or hotter for ten or more days, or 60° C. to 70° C. for four or more days, before detection of a voltage drop, and presence of electrically conductive foreign bodies in the lithium ion battery is determined upon detection of a voltage drop larger than a predetermined voltage drop reference.
Japanese Patent Laid-open Publication No. 2005-243537 (Patent Document 2) discloses a method for suppressing occurrence of minute short-circuiting between a cathode and an anode by initially charging a lithium ion secondary battery which is a nonaqueous electrolyte secondary battery to 0.01% to 0.1% of the battery capacity to thereby set the anode potential to 1.5 v or larger under the Li/Li+ reference and the cathode potential to 3.5 v or larger under the Li/Li+ reference, and then allowing the battery to stand for one to 48 hours.